The present invention relates to intermetallic materials based on MoSi2, intermetallic matrix composites and methods of making the same. More particularly, the invention is directed to a MoSi2 based material having an engineered micro-structure provided through the use of in-situ reinforcement whiskers.
MoSi2 is an attractive intermetallic for structural applications due to its excellent high-temperature oxidation resistance, low density and high thermal conductivity. However, it is brittle at low temperatures, weak at high temperatures and suffers from accelerated oxidation at intermediate temperatures. The accelerated oxidation of MoSi2 at intermediate temperatures causes the material to disintegrate into powder, a phenomenon known as pesting.
Pesting is a general term describing the catastrophic oxidation of intermetallic materials at intermediate temperatures. The accelerated oxidation leads to the disintegration of the material and component failure. For MoSi2 the temperature at which pesting is most pronounced is approximately 500xc2x0 C. It has been observed that at 500xc2x0 C. bulk (i.e., non-composite) MoSi2, as well as, composites of MoSi2 with alumina and aluminum nitride also suffer total disintegration within relatively short time periods, e.g. 100 hours.
The pested samples yield powdery products consisting of MoO3 whiskers, SiO2 clusters, and residual MoSi2. The MoO3 whiskers exhibited protruding characteristics and were concentrated at the grain boundaries and cracks. The pesting phenomenon in MoSi2 has been concluded to be the result of the formation of voluminous molybdenum oxides in microcracks. While not wanting to be bound by theory, the accelerated oxidation apparently involves the simultaneous formation of MoO3 and SiO2 in amounts essentially determined by the Mo and Si concentrations in the intermetallic.
The addition of about 30 to 50 volume percent of Si3N4 particulate to MoSi2 reduced the pesting by forming a protective oxide scale as disclosed in assignee""s related U.S. Pat. No. 5,429,997, the teachings of which are hereby incorporated by reference. In addition, improvements in room temperature fracture toughness, reductions in the 1200xc2x0 C. compressive creep rates and lowered coefficient of thermal expansion were attained. Additional improvements in toughness and elevated temperature strength were achieved by reinforcing the MoSi2xe2x80x94Si3N4 matrix with about 30 volume percent of silicon carbide continuous fibers. The use of fiber reinforcement is not entirely satisfactory due to the high costs of the present state-of-the-art techniques for making fiber reinforced, composites.
A further difficulty with the use of fiber reinforcement is the coefficient of thermal expansion mismatch between MoSi2 and most potential reinforcing materials. MoSi2 has a relatively high coefficient of thermal expansion as compared to most potential reinforcing materials such as silicon carbide fibers. The coefficient of thermal expansion mismatch between the fiber and the matrix material tends to result in matrix cracking during fabrication and severe matrix cracking during thermal cycling which in turn results in component failure. Possible reinforcing fibers include high strength ceramic fibers such as silicon carbide, single crystal alumina, and ductile, high strength molybdenum and tungsten alloy fibers. Ductile niobium fibers have shown improvements in low temperatures strength and toughness, but a severe reaction between the fiber and MoSi2 limits its use and, in any case, it does not provide improved high-temperature characteristics. The addition of silicon carbide whiskers has yielded improvements in room temperature toughness, but pesting and coefficient of thermal expansion mismatch continue to be problems.
It has now been discovered that a MoSi2 based material may be provided with an engineered micro-structure through processing and composition control. In-situ reinforcement of whisker type xcex2-Si3N4 grains in a MoSi2 matrix offers a unique combination of attributes. The in-situ reinforcement is believed to provide a more tortuous crack path with elongated grains or whiskers which lead to crack bridging and deflection resulting in very high fracture toughness.
The invention contemplates a new MoSi2 based alloy composition which exhibits excellent pest resistance at low temperatures (400 to 600xc2x0 C.), good coefficient of thermal expansion match with potential fiber reinforcement, excellent oxidation resistance at elevated temperatures and high fracture toughness enabling its use as a monolithic material. Accordingly, a MoSi2-based matrix contains a high-volume fraction of randomly oriented in-situ grown long whisker type grains of xcex2-Si3N4 in the MoSi2 matrix. The matrix is characterized in part by a toughened micro-structure, lower density, lower coefficient of thermal expansion, excellent resistance to pest attack, and it is much stronger than the binary MoSi2 material.
The invention also contemplates processing conditions to achieve fully dense alloys with engineered microstructure through the use of sintering aids to grow the long whiskers of xcex2-Si3N4 in the matrix. During processing, high-temperature and pressure conditions are used to convert the xcex1-Si3N4 particles to randomly oriented xcex2-Si3N4 long whiskers. Suitable sintering aids include rare earth oxides.
It is presently believed that the improved pesting is related to the formation of more protective silicon oxy nitride, Si2ON2, and/or mullite/SiO2 oxide scales that suppress the formation of non-protective MoO3. The invention also contemplates forming an outer protective layer of Al2O3xc2x7SiO2 followed by an inner layer of SiO2 for better oxidation resistance in reducing or low partial pressure oxygen atmospheres.
The invention also contemplates the use of the MoSi2-xcex2Si3N4 as a matrix in a ceramic fiber reinforced composite to achieve high specific strength, high first matrix cracking stressed and toughness without exhibiting any pesting or cracking during long-term thermal cycling at high and low temperatures. Silicon carbide fibers comprise a preferred reinforcing fiber.
According to a first preferred composition of the invention, there""s provided MoSi2 based matrix materials containing at least about 20 percent by volume xcex2-Si3N4 whisker type grains based on the combined volume of the MoSi2 and the xcex2-Si3N4. More preferably, the compositionof the invention comprises from about 30 to about 50 percent by volume xcex2-Si3N4 based on the combined volume of the MoSi2 and xcex2-Si3N4.
The achievement of the whisker type xcex2-Si3N4 grains is enhanced by the use of rare earth oxide sintering aids. The rare earth oxides are nano sized. Preferred rare earth oxides include Y2O3 and Al2O3. The sintering aids are used in amounts ranging from 2 to 6 percent by weight based on the combined weight of the MoSi2 and xcex2-Si3N4.
A preferred fiber reinforced composite comprises the above noted MoSi2-xcex2Si3N4 composition as a matrix material and ceramic reinforcing fiber. The reinforcing fiber is interspersed with the matrix material. The matrix material comprises at least about 50 percent, and more preferably about 70 percent by volume of the composite based on the combined volume of the matrix material and the xcex2-Si3N4 reinforcing fiber. In more preferred arrangements, the reinforcing fiber is a silicon carbide fiber, and it is present in the matrix in an amount of about 30 percent by volume based on the combined volume of the matrix and the reinforcing fiber.
In accordance with the method of the present invention and preferred processing, mixtures of MoSi2 and Si3N4 are blended and milled to micrometer particle size with the prior addition of sintering aids. The mixtures are formed into thin sheets or plates using vacuum hot pressing to achieve a relatively high green density. This first stage is followed by hot isostatic pressing to achieve full density as well as growth of whisker type grains of xcex2-Si3N4. This two stage processing with the use of sintering aids enables reduction of the maximum heating temperature without reduction of the final level of densification.